Introduction to Viruses

How Were Viruses First Discovered? 1

In the late 1800s, filter apparatuses were devised that could remove bacteria (the smallest known organisms), from a liquid. It was not known at that time that smaller infectious agents existed that could pass through the filter. The Russian scientist, Dimitri Ivanowsky, studying the plant tobacco mosaic disease (TMV*), which affect plants, discovered that when he applied sap from infected tobacco leaves to the filter, the liquid that passed through still was infectious. However, he did not recognize that he had discovered a new type of infectious agent.

This insight was obtained six years later by the Dutch scientist, Martinus Beijerinck, who discovered that the infectious agent which passed through the filter could reproduce but would not grow on Petri dishes used to cultivate bacteria. He further realized that these agents required the presence of a host cell to reproduce. He named the agent responsible for tobacco mosaic disease a virus, after the Latin term for poison. He thought that this agent must be much smaller and simpler than bacteria. The subsequent crystallization and electron microscope images obtained by the American scientist, Wendell Stanley, in 1935 confirmed this, and the agent was named tobacco mosaic virus.

*Tobacco mosaic virus causes stunted plant growth and mottled, discolored plant leaves, especially in tobacco and other members of the tomato family (Solanceae).